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Tour Engine has Prototype II split-cycle engine running

Prototype II Tour Engine—a novel split-cycle engine—on the bench. The hot side is on the right. Click to enlarge.

Tour Engine, the developer of a novel split-cycle engine (earlier post), has its Prototype II engine running and will present details on its operation at the upcoming SAE 2012 World Congress in Detroit.

In early bench-testing of the new prototype, Tour found samples of a similar work gain value to that from a conventional control engine (129 kJ for the control and 124 kJ for the Tour engine in the example depicted below), with both operated at about half throttle and using a symmetrical Tour engine with a compression ratio similar to the expansion ratio (8:1). Other examples show similar trends, according to Tour.

However, notes Oded Tour, the Tour engine can be greatly improved by having an expansion ratio of up to 3 times larger than the compression ratio and by further differential engineering of the compressor cylinder and the combustor cylinder. Thus, the company plans in the near future to modify prototype II to have an expansion ratio of 16:1 with compression ratio of 8:1.

At first cut, roughly, we have not done any worse than the baseline engine. The way forward for the Tour engine is clear substantial improvement.

—Oded Tour

Recordings from Prototype II. Top: Five cycles showing in-cylinder pressure as a function of time. Middle: A set of readings relating to a specific cycle. Bottom left: Pressure as a function of volume for the same specific cycle (p-V): The area within the blue and red curves represents the compression work invested and the resulting combustion work extracted, respectively. Bottom Right: 1) At power piston TDC the crossover valve opens and the pressures in the two cylinders are almost equalized. 2) Timing of the spark. 3) Combustion initiation is timed to compound the maximum pressure achieved during compression (with an open crossover valve). 4) Closing of the crossover valve. 5) Power stroke is being executed in the Hot-Cylinder. Click to enlarge.

Readings from control engine. Click to enlarge.

The premise of a split-cycle engine is that segregating the intake and compression strokes in one cylinder, and the combustion and expansion strokes in another, coupled cylinder, provides a thermodynamic advantage enabling a more efficient engine. Most current split-cycle designs use a gas crossover passage or intermediate chamber to connect discrete cylinder pairs. By contrast, the Tour engine configuration directly couples the two opposing cylinders, with a single crossover valve controlling the charge flow between the two cylinders.

SolidWorks Design of Prototype II, partial transparent front view. The vertical purple part located between the two engine sides is the custom designed connecting plate that hosts the crossover valve. The hydraulic pump that is connected via a timing belt to the engine and is used to load the engine is depicted on the top. Click to enlarge.   SolidWorks Design of Prototype II, back view. The four gearwheels have the following functions: At the 9 o’clock position is the gearwheel connected to the compressor cylinder. The crossover valve mechanical cam is at 12 o’clock (since the valve actuation is both precise and fast – it opens and closes within about 45 degrees so that a large cam is required). At the 3 o’clock position is the combustion cylinder gearwheel, and at 6 o’clock is the hydraulic pump gearwheel. Click to enlarge.

The crossover valve enables the execution of an integrated cycle: the inducted working fluid is compressed and combusted as part of a single cycle, thereby avoiding piston runaway. The Prototype II Tour engine, based on two 190 cc Briggs & Stratton engines, uses a mechanically actuated crossover valve.

The Tour engine is designed to operate using conventional realistic compression ratios (8:1 to 20:1 depending upon fuel type and the use of SI or CI cycle), and is designed to fire at the end of the compression process (before any decompression occurs)—while the crossover valve remains open—very similarly to conventional engines but retaining the split-cycle thermodynamic advantages.

Firing with an open crossover valve allows the TourEngine to follow the conventional 4-stroke cycle thermodynamics, but on a split-cycle platform. (The disadvantage is a small efficiency penalty associated with a larger surface area acting as a heat sink at combustion initiation.) With complete charge transfer, the crossover valve closes; combustion continues in the hot cylinder.

The crossover valve is key to the success of the engine; it must be able to open to allow the compressed charge transfer and then immediately close (on the order of 30–50 crankshaft degrees) and transfer the charge with minimal resistance. In other words, the valve needs to be large enough in cross-section not to be a bottleneck, but also thin enough in profile to ensure minimal dead volume.

Dead volume on the compression side prevents full transfer of the compressed working fluid, while dead volume on the expansion side reduces volumetric efficiency and decreases the phase lag for a given compression ratio, which will require even faster valve actuation and therefore will be more challenging

Crossover valve and cylinder connecting plate. The engine is designed in a modular fashion such that several different connecting plates housing different crossover valves could be tested on the same engine. Click to enlarge.

The alpha prototype used a spring-loaded crossover valve. In addition to the current mechanically actuated crossover valve used in Prototype II, Tour is also developing several other crossover valve concepts including an electromagnetic crossover valve that is actuated by compression (open) and combustion (close) while the electromagnetic force is used to fine tune (hinge) the valve to close and open at the precise timing.

According to Oded Tour, Tour Engine is in discussions with several OEMs on establishing a joint development aiming on taking the concept to the next level by building an advanced Tour engine.

GM Vice President of Global Research and Development Dr. Alan Taub noted in his talk at the 2011 DEER conference that split-cycle engine technology looks promising and that GM was pushing in its R&D laboratory to see if it can get split-cycle technology “moving”. (Earlier post.)

...we may finally be entering the era where the demand for fuel efficiency will be allowing us to break away from what has become the standard architecture of our engines, and in particular the idea of separating the compression and the combustion (expansion) cylinders, into a dual stage engine. People have talked about it, a lot of people are starting to build prototypes in this, and the driving force is clear: we can see very dramatic improvements in efficiency by going to the DCDE [Discrete Compression Discrete Expansion] architecture. It takes mass, it takes cost, it takes complexity, but giving those kind of efficiency improvements, it is definitely something we need to explore further.

—Alan Taub at DEER 2011

Dr. Chris Atkinson, Professor, Mechanical and Aerospace Engineering at West Virginia University and an advisor to Tour Engine, concurs that over the last three to four years, “a remarkable openness to contemplating such new engine architectures” has emerged among the major OEMs. As for Tour Engine, he added, leaving aside the commercialization aspects:

They have done remarkably well from a technical point of view. To have a [new] engine running legitimately with comparable efficiency to a conventional engine is very much a remarkable feat. Normally, you take several steps back and then you try to work out what you’ve done wrong, whereas here, they are close to conventional already. On a shoestring budget with a minimum of people they have done remarkable things; the quality is very good—approaching OEM quality.

The Prototype II Tour Engine was built with the aid of the Israel Ministry of Energy and Water Resources.



@Anne & Engineer-Poet
Yes, I have provided the link several times and the report is for free. Below, you will find it once more. Page 116 is all you need to look at.


So, what about intercooling? It is about time for you to admit that you were wrong.


Intercooling decreases efficiency but allows to put more charge in an engine. Combined with heat recuperation however in some thermodynamic cycles allows to increase efficiency. As far as I remember, intercooling alone decreases thermodynamic efficiency and E-P is right.


Thank you, Simon.

Page 116 is all you need to look at.
You mean that's all you want us to look at.  If you dig into the data you'll find Figure 56 on page 120 which shows that at least some of the assumptions are bogus; the BEV powered by NG-fired electric capacity supposedly emits ~85 g/km CO2, but a PHEV taking electricity from the same source hits 75 or so.  When CCGTs are operating at 60% thermal efficiency (LHV) and ICEs are around 40%, it is simply incredible that the ICE running on a higher-carbon fuel can beat the BEV's power source running on natural gas.  The BEV running on NG CCGTs has roughly a 2:1 advantage between the carbon-intensity of the fuel and the efficiency of the prime mover.

I'd dig deeper into the calculations but I don't have time at the moment.



Here is the "Heywood"/Kromer paper, no paywall:

On p.76 it says

QUOTE "The net result of these trends – in particular, the changing grid mix shown in Table 30 and the improved efficiency shown in Table 29 – is that the emissions rate of the electric grid does not
change substantially over the next several decades according to the EIA estimates. This is because the increasing efficiency of the grid over time (as old units are replaced with newer ones) is offset by the increasing share of coal within the overall grid mix. The base case
assumptions give an average grid emissions rate of 635g CO2/MWh generated for the 2030 grid, as compared to 640g CO2/MWh in the present." ENDQUOTE.

As I have also shown in a previous thread, a Prius causes less CO2 than a Leaf, with the current Grid Mix. And as you can see from the above paper, this will still be the case in 2030.

Not to Anne in particular:

Please nobody think that "they" are "out to kill the electric car, just like they did with the EV1 in the 1970s". What we want to do is to pick the technology that results in the lowest consumption and the lowest CO2 emissions overall. Right now that technology is a hybrid-diesel car, and other hybrid technologies will continue to evolve. It will be many years before an electric car is cleaner than a a hybrid-diesel/whatever-is-the-best-engine car.


Correction: The last comment was in response to @Engineer-Poet first and @Anne second.

And as you can see it was in support of what @Peter XX said.

And please pardon the sloppy typing, I'm not very good at it.


Do you really believe your own statement? Intercooling increases efficiency. This was proven by Ralph Miller already in the 1930's. My recommendation is that you have a look in the literature. If you still have some questions after that please come back and I can explain to you in more detail why intercooling is such a nice feature. Recall, that practically any engine manufacturers use it on turbocharged engines.

Thanks for your support!

For those who are not familiar with prof. John B. Heywood, I can mention that he is the one of the most experienced and competent expert in engine and drivetrain technology in the USA. The study I am referring to (albeit that there are several other similar studies from MIT) is the largest and most comprehensive study in this field. It has also been posted in the news on GCC, so it should not be unfamiliar. For those of you who still are in doubt and might claim that this is just a university report, I might add that a paper from this study has also been peer-reviewed and published by SAE.


"I can explain to you in more detail why intercooling is such a nice feature. Recall, that practically any engine manufacturers use it on turbocharged engines."

Both turbocharging and intercooling allow for more charge in an engine. This can make an engine lighter and perhaps that way by weight reduction allow to increase FUEL EFFICIENCY somewhat more than the SLIGHT thermodynamic DECREASE in CYCLE EFFICIENCY as E-P rightly claims. I do not however consider it impossible for ENGINE EFFICIENCY to increase which you claim as engine efficiency is completely different (cylinder wall heat losses during compression for example will be lower with intercooling, so it is a complex issue) from thermodynamic cycle efficiency or carnot efficiency for that matter. You may both be right in a way.


Such effects are probably slight anyway, from what I recall the decrease in ideal thermodynamic cycle efficiency from intercooling is not that big so differences in for example cylinder wall heat losses could compensate somewhat. Either way, either the increase or decrease will be be small which is why it is not disabandoned for fuel efficiency reasons and there are other good reasons than fuel efficiency (which few people care about an extra percent more or less anyway) to use intercooling.


No, EP is wrong again although this time he might be aware of it. I will give you the short version this time:

With intercooling, the compression work decreases considerably (for a given amount of air mass). Furthermore, heat losses in the cylinder decrease. At the same specific power, the cylinder pressure decreases (although one might want to increase power) with some reduction in engine friction as a consequence. There are also positive effect on combustion with less irreversibilities and reduced dissociation. Since engine specific power can be increased, downsizing can be utilized to also increase efficiency at light load. For gasoline engines, the compression ratio can be increased and less fuel enrichment is needed at high load. More EGR can be used in many cases with additional benefits for fuel consumption and emissions. Speaking about emissions, one substantial advantage on diesel engines is the reduction in NOx emissions.

On the negative side there is a small increase in pressure drop over the intercooler and less energy for the turbine. The former is reduced by improved intercooler design and the latter is partly overcome by new turbine matching. All in all, however, there is a significant net increase in engine efficiency with intercooling. For example, when intercooling was introduced in heavy-duty engines, the fuel consumption was reduced by at least 5 %. Although this might seem as a small improvement, recall that the annual improvement is at about 0.5 %. Thus, this was equal to about 10 years of development and it was one of the biggest steps ever made in engine development.


While I'd be the last to argue that an intercooled engine cannot be more efficient than one without intercooling (just the downsized mechanical section and reduced compression back-work are solid advantages), what Peter XX said to start this little brouhaha is preposterous.  He claimed that cooling after compression, immediately before combustion improves efficiency.

Intercooling is still a loss.  It doesn't mean it can't be outweighed by reduction in other losses.


That is increased engine efficiency, which I don't doubt is possible. Thermodynamic cycle efficiency is still less. There is less work in compression but that means you end up with a lower temperature after compression and will need to add more fuel to get a temperature rise to get the same work from expansion. Net effect in thermodynamic cycle analysis is a decrease in efficiency. But anyway, a gasoline engine does not come even close to the ideal Otto (constant volume heat addition) cycle efficiency nor does a diesel come close to the ideal constant pressure heat addition cycle. Thermodynamic cycle efficiency is idealized, what many don't know is that it does not even produce power (since it is infinitely slow) and hence does not take things like part load into account (as load means power).


I also don't doubt anything you said apart from thermodynamic cycle efficiency. I did not know the figure for fuel consumption reduction was about 5%. I thought peak thermal efficiency was lower by 2% or less. Do you have figures for that?


I am not sure I understand your question but if I do, the explanation is trivial. There is a proportion between specific fuel consumption and efficiency. If specific fuel consumption is lower, efficiency is higher. One cannot break the laws of nature. Suppose that the peak efficiency is 42%, as for the HD engine in my example. If you reduce the fuel consumption by 5%, you get an efficiency of 44.2% (42/0.95=44.2). It is the same factor, 0.95 in both cases. As said, if you reduce fuel consumption, the efficiency must be higher. Of course, in “absolute” numbers the difference is -2.2% for the efficiency.


E-P: after reading his post thoroughly, it seems Peter_XX interpolated from INTERcooling, where most of the compression is done in the cylinder anyway, that COOLING after compression could increase efficiency too due to what he assumes is better combustion efficiency. I find this highly dubious as it is indeed a big loss, you don't gain anything from reduced heat losses to the cylinder wall during compression and what you gain from combustion efficiency is probably quite little. I don't recall figures for combustion efficiency differences between diesels and gasoline engines but a diesel is regardless of its higher peak combustion temperatures and condequent lower combustion efficiency and more constant pressure like heat addition STILL considerably more efficient than a gasoline engine due to compression ratio alone. I'd bet on it being a considerable net loss despite improved combustion efficiency.


I assumed when you mentioned fuel efficiency as on a certain cycle like NEDC. Such a cycle is not at optimum efficiency. I thought peak thermal efficiency of an intercooled engine was usually somewhat less (2% or so) due to the reduced thermodynamic cycle efficiency outweighing the benefits from reduced cylinder wall losses during compression etcetera. Anyway, I must have recalled that wrongly as you can't really compare different engines/engine designs "on average".


First, you should realize that I have worked with engines and emissions during whole my adult life so I know what I am talking about. I do not post any data or explanation that is not true. You can be absolutely sure that my comments about intercooling are correct. Among engine developers, the advantages of intercooling are well-know and not disputed. You can be absolutely sure that if engine efficiency would increase by removing the intercooler, engine manufacturers would do that immediately. I already gave you some brief explanations about the impact of intercooling but they should be sufficient. You cannot focus on one single effect of those I mentioned but you should realize that the sum of these effects is positive.

Heavy-duty engines are dominated by high engine load. Average engine efficiency in a representative driving cycle is often at around 40%. Thus, the 5% gain in fuel consumption at peak efficiency is also valid for real-world driving. If we take another extreme, e.g. a gasoline engine used in city driving, the main advantage of intercooling is the increase in compression ratio. You must realize that an increase in compression ratio by two units, e.g. from 8:1 to 10:1 has a decisive impact on fuel consumption in this case. Thus, the gain in fuel consumption in a representative driving cycle for this engine type can be even higher than the previously mentioned case.

One interesting option is to use both intercooling and aftercooling with two-stage turbocharging. (There are some issues to discuss about nomenclature regarding Intercooling/aftercooling but I will leave this to later discussions.) This is already used on heavy-duty engines and recently, the GM 2-liter diesel engine also employed this feature. Other manufacturers will probably follow.

Finally, I have to add one more clarification. This is about the gain in compression work with intercooling. If you have a higher temperature, you also have lower density and thus, the pressure must be increased to compensate for that so that you get the same air mass in the cylinder. Therefore, the compression work is higher when intercooling is not used. If you reduce the compression ratio to compensate (i.e. reduce compression work in the engine cylinder), you lose efficiency due to less expansion work.


Peter XX continues to obfuscate and backpedal from his previous statement.

Intercooling can indeed improve efficiency, because it helps to get closer to the Carnot cycle's isothermal compression.  But in the Carnot cycle, isothermal compression is followed by isentropic compression followed immediately by isothermal expansion; the temperature of the working fluid is not reduced immediately before adding heat.

Peter, just admit you got something wrong.  The burden of acting like an infallible oracle is going to get to you otherwise.


I was not around some 75 years ago when Ralph Miller convinced engineers that intercooling was beneficial for efficiency. When I started to work in the 1980’s engineers were convinced. Both intercooled and non-intercooled engines were in production then, so we could easily compare the efficiency and fuel consumption. Now, we have a situation when a Poet does not want to recognize this fact. However, this time I will show no mercy! I do not care about your home-cooked theories but I challenge you to post a diagram proving your point. I already have such a diagram ready but I will not post it until you come up with some evidence for your hypothesis, or else, admit that you were totally wrong this time. You make me laugh…

this time I will show no mercy!
Oooh, I'm quaking in my boots sneakers again.  Wait, no I'm not.
I challenge you to post a diagram proving your point.

Note the lack of any heat extraction during the compression from point 4 to point 1.


Ha, ha, this gets more and more fun... You try to prove your point by showing a pv-diagram of a Carnot cycle. Diesel and Otto cycles are very far from Carnot cycles. Anyway, if we neglect this fact, I can use your own Internet source and some simple calculations to prove that you are wrong; even for a Carnot engine.

The following is a very simplified calculation: We can use 1000 K as maximum temperature in both cases and 150°C (423 K) for the engine without intercooler and 50°C (323 K) for the engine with intercooler. The latter temperatures will be the inlet temperatures for this imaginary engine. The idealized efficiency for the engine without intercooler will be 57.7% (1-1000/423.15) and 67.7% (1-1000/323.15) for the engine with intercooler. A significant advantage for the engine with an intercooler! As I previously said…

I am stunned that you are not able to make such simplified calculations as the one above.

Anyway, I still have my real-world comparison of actual production engines to show you. I will just have to find an Internet site to upload a diagram…


So, now it is time for some hard facts. As I said previously, I was stunned to see that you had no clue about how an intercooler works. All engine engineers who have worked in this field know the positive impact that an Intercooler has on efficiency. Obviously this is new information for you, so my conclusion is that you have no experience from engine development whatsoever.

Intercoolers were introduced on heavy-duty vehicles a long time ago. One of the latest to adopt this technology was the Swedish truck manufacturer Scania. They also continued with non-intercooled engines in parallel to intercooled engines, usually on lower rated engines, for many years. This makes a comparison of Scania engines ideal. I have chosen two engines, one without an intercooler (DS11 01) and one with intercooler (DSC11 01). The timeframe is 1982. Scania had three engine families in production at that time (9, 11 and 14 liters) but the 11-liter engine represented more than 50% of the production, so it is a very relevant comparison. The Internet site that I uploaded this comparison on “shaved” the graph on both left and right hand sides, leaving out the text on the y-axis. It is brake specific fuel consumption (BSFC) in g/kWh on the y-axis. As you can see, the relative difference is 5-6%. The intercooled engine has lower fuel consumption than the non-intercooled engine at all engine speeds, just as I indicated previously. Needless to say, power and torque was higher for the intercooled engine. Furthermore, the rated speed (2000 vs. 2200 r/min) is lower, which also would enable “downspeeding” in addition to “downsizing”.

Let this be a lesson to you: Never ever question my statements again. I always have hard facts to back up my statements. Or, maybe you do not believe in these data? Do you want more in-depth technical explanations? I can provide that. Do you want more evidence? Well, I also have that. Just let me know what you need.


At last you post a diagram... which is not relevant to the claim you made.  I've got some frozen brakes to deal with, so I'll have to put the task of ripping you a new one off until this evening.


Well, you better give up now!!! Of course the diagram is absolutely relevant! It is an intercooled engine vs. a non-intercooled engine. It proves that I am correct. I said 5%, didn’t I? This was what you asked for. Do you want me to calculate from g/kWh to efficiency? I suppose you are not capable of doing that calculation either.

Until now you have shown nothing, absolutely nothing. This is a walk-over victory for me! I you have some sense left, you should give up now!

In case you did not notice, I made a typing error in one of the previous comments, the correct typing should be: 1-423.15/1000=57.7% and similarly for the other case.


I have now uploaded a diagram on the engine efficiency for the two engines in the previous comparison I made. Maybe you think that this comparison is more relevant, although it again proves that you were wrong and that I was right. The relative difference is of course the same as for BSFC. In “absolute” percentage numbers, the difference is roughly 2.5%.

Scania does not provide any information on the energy content of their test fuel. I have anticipated an energy content of 42.65 MJ/kg, which is a normal level for diesel fuel, in the calculation from specific fuel consumption to efficiency.

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